Several features have been identified as desirable by users of sanitary napkins. Among these features are a dry surface adjacent the body and a napkin which is more comfortable than the traditional napkin after substantial amounts of fluid have been absorbed.
One of the difficulties inherent in a conventional sanitary napkin made of multiple layers of cellulosic material is that when fluid is absorbed by wood pulp fluff or similar cellulosic material the capillary walls tend to collapse inward. This collapse prevents fluid from being conducted downward through the capillaries and also substantially diminishes the inherent resiliency in the cellulosic material making the napkin relatively hard and uncomfortable when compared to its dry state.
A related problem is that after the capillaries collapse, fluid which would normally be conducted downward tends to remain at or near the top surface of the napkin and either rewets or fails to penetrate the cover providing a wet uncomfortable napkin surface.
Of course, the collapse of the capillaries in the uppermost portion of a cellulosic absorbent system prevents adequate utilization of the bottom portion of the absorbent because fluid fails to reach this area.
The problems of diminished comfort and downward direction fluid conductivity have been recognized and one of the proposed solutions has been to introduce thermoplastic fiber into an upper layer of a multilayer absorbent matrix. Examples of patents which teach the addition of thermoplastic material as an independent layer or as an admixture with conventional cellulose are U.S. Pat. Nos. 4,082,886 and 4,129,132 issued to George A. M. Butterworth et al; 3,976,074 issued to Harry G. Fitzgerald et al; 4,054,141 issued to Julius Schwaiger et al; 4,047,531 issued to Hamzeh Karama; 3,545,441 issued to Gunnar Gravdahl and 4,219,024 issued to Donald Patience et al.
While the presence of thermoplastic fibers either in combination with cellulosic fibers or by itself in the layer adjacent the cover adds resilience and minimizes a cell wall collapse, fluid conduction downward through this layer is minimized due to the enlarged capillary size present within the batt containing the thermoplastic fiber. While napkin constructions which utilize a thermoplastic fiber in an upper portion of an absorbent batt tend to feature conventional cellulosic absorbent materials below, the distance between the smaller capillaries present in the cellulosic portion and the cover substantially diminish the capillary attraction required to conduct the fluid downward. Relatively small amounts of fluid therefore are taken up by the principal absorbent component which is located towards the center of the napkin and the fluid tends to remain at or near the cover surface producing not only a napkin of substantially diminished capacity, but one with a wet, uncomfortable, top surface as well.
U.S. Pat. No. 3,143,113 by Victor Mills discloses a sanitary napkin containing multiple layers of cellulosic absorbent material with the uppermost layer formed from a web which has been turned 90.degree. from its forming axis. This upper layer of absorbent is designed, according to the inventor, to help spread the fluid longitudinally.